There are still many people using older systems, so your policy of not supporting them is unreasonable, as you are losing customers because of it. I had to use a courier service for this reason. Therefore, I request that you reconsider supporting older systems that are still widely used. It is in your own interest to do so; otherwise, people will be forced to go elsewhere.

This is what had happened:

I could not send an item on-line, because I got the following error when trying to sent through the web page: https://send.royalmail.com/ :

"Unexpected Application Error!
n.costs.toSorted is not a function
[...]"

Your customer support said that:
"we have had a number of customers reporting issues when running on
older operating systems, such as Windows 7, Windows 8, Windows 10 and older Mac OS X versions. These are no longer supported or updated by Windows/Apple.

Using an outdated operating system can mean compatibility issues, as browsers and other software will continue to update to meet current system standards.
This means the site may start to not work correctly, or stop working at all.
This is likely to be the cause of the issues you are seeing. We would
recommend updating to a supported operating system as soon as possible to
avoid any further issues."